In the nucleus, hnRNA is complexed with a set of proteins and condensed into the "beads-on-a-string hnRNP structure. The physical basis for the beaded appearance of the hnRNP is not understood, even in the most general terms. The major aim of the proposed research is a detailed understanding of this structure, with a long range goal of determining how the structure is involved in hnRNA processing and transport, and how these processes are controlled. The hnRNP structrual proteins are being purified by standard biochemical methods, and are assayed by their ability to bind to single-stranded nucleic acids, generating a beaded structure along the nucleic acid molecule. The protein-protein and RNA-protein interactions of the purified proteins are being analyzed by a combination of electron microscopy and physical methods. Antibodies to the purified proteins are also being prepared as structural probes. A protein (HD40) has recently been isolated from A. salina messenger RNP and purified to greater than 95 percent homogeneity. When the pure protein is added to RNA, a beaded structure is formed with properties similar to those of the hnRNP complex. The physical basis of the HD40-RNA interaction is being investigated as a part of this project with the expectation that it will provide additional insight into the RNP structure, and may reveal the existence of a fundamental, but as yet unrecognized class of RNA-protein interactions.